A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The great majority of today's rigid-frame bicycles have a frame with upright seating. Such upright bicycles generally feature the so-called diamond frame, a truss consisting of two triangles: the front triangle and the rear triangle. In a conventional diamond frame, the front “triangle” is not actually a triangle because it consists of four tubes: the head tube, top tube, down tube and seat tube. The head tube contains the headset, the set of bearings that allows the front fork (which supports the front wheel) to turn smoothly for steering and balance. The top tube connects the head tube to the seat tube in the vicinity of the top of the seat tube, and the down tube connects the head tube to the bottom bracket. The bottom of the seat tube is also attached to the bottom bracket
The rear triangle consists of the seat tube and paired chain stays and paired seat stays. The chain stays run roughly parallel to the chain, connecting the bottom bracket to the rear fork ends (which support the rear wheel). The seat stays connect the top of the seat tube (at or near the same point as the top tube) to the rear fork ends.
Many modern bicycles do not utilize a diamond frame, for example because: the frame is constructed in such a way that it does not consist of tubes attached one to another (for example, frames made of composite materials); or the frame involves a rear suspension system permitting rearward components of the bicycle (e.g., the rear wheel) to move relative to other components of the bicycle (e.g., the seat); or both. However, the terms used to describe the members of a conventional diamond frame (being, head tube, top tube, down tube, seat tube, chain stays and seat stays) are often used to describe analogous features on non-diamond frames and are at times so used herein.
Most bicycles use a chain to transmit power to the rear wheel. The drivetrain includes pedals which rotate the crank arms, which are attached to a spindle that rotates within the bottom bracket. With a chaindrive, a chainring attached to a crank arm drives the chain, which in turn rotates the rear wheel via a rear sprocket. The crank arms, chainrings and spindle are commonly referred to as the crankset. Most chaindrive systems have some form of gearing, typically comprising multiple rear sprockets of different sizes, multiple chainrings of different sizes and user controllable devices (referred to as derailleurs) for moving the chain between rear sprockets and between the chainrings, so as to selectively vary the gear ratio.
In chain drive systems, the portion of chain extending between the top of a chainring and the top of a rear sprocket conveys the motive force from the pedals to the rear wheels. When the rider is pedaling, this top portion of chain is under tension. In a bicycle without a rear suspension, this chain tension is resisted by the rigid frame (e.g., the rear triangle) to which the rear wheel is mounted.
In a bicycle with a rear suspension system, some portion of the force of such chain tension may be imparted to the suspension system. As well, movement of the rear suspension system relative to the bottom bracket may dynamically tension or slacken the portion of chain extending between the top of a chainring and the top of a rear sprocket, thereby affecting the pedaling resistance experienced by the rider. The direction of the force conveyed along the portion of chain extending between the top of a chainring and the top of a rear sprocket is referred to as the chain line. A further complication is that bicycles typically have multiple chainrings and multiple rear sprockets so as to provide rider selectable gear ratios; in the result, most bicycles do not have a single chain line, but rather have multiple chain lines.
A bicycle suspension is the system or systems used to suspend the rider and all or part of the bicycle in order to protect them from the roughness of the terrain over which they travel. Bicycle suspensions are used primarily on mountain bikes, but are also common on hybrid bicycles, and can even be found on some road bicycles. Bicycle suspension can be implemented in a variety of ways, including: front-fork suspension and rear suspension. It is not uncommon for a mountain bike to have front suspension but no rear suspension (such a suspension configuration is often referred to as a hardtail). However, it is uncommon for a mountain bike to have a rear suspension system but no front suspension system. Thus, rear suspension systems on mountain bikes are typically part of a full suspension system.
Suspension systems for mountain bikes first appeared in roughly the early 1990's. Over the ensuing years developers and users of mountain bike suspension systems recognized a variety of factors affecting suspension performance and general riding performance of suspension system, which factors are interrelated in dynamic and complex ways. It was soon realized that the fact that bicycles are powered by human effort means that effects on the drive train caused by suspension system movement that would, in the case of engine driven vehicles, be minor or unnoticeable, are significant in bicycles.
In the field of bicycle suspension systems, the following terms are generally used as follows:                Travel refers to how much movement a suspension mechanism allows. It usually measures how much the wheel axle moves.        Squat refers to rear suspension compression due to acceleration of the bicycle caused by rider pedaling (i.e., as opposed to gravity induced acceleration when coasting down an incline) and the resulting “weight transfer”. Weight transfer occurs because the center of gravity of the bicycle and rider (typically located within the rider) is displace from the location where the acceleration force is applied (i.e., where the rear wheel contacts the riding surface). Thus, under powered acceleration, the rear wheel carries more weight (and the front wheel carries less weight) than if the bike was rolling at constant speed. If the bike has rear suspension, then the extra weight carried by the rear wheel (due to weight transfer) tends to cause the rear suspension to compress. Weight transfer is unavoidable during acceleration, and occurs equally with all suspended and non-suspended vehicles undergoing acceleration.        Pedal bob (also, kickback or monkey motion) refers to repeated squat and rebound with each pedal stroke. Pedal bob reduces the efficiency of, or interferes with, a rider's pedal stroke—especially when climbing up steep hills.        Pedal feedback, or chainstay lengthening, refers to torque applied to the crankset by the chain caused by motion of the rear axle relative to the bottom bracket. Pedal feedback is caused by an increase in the distance between the top of the relevant chainring and the top of the relevant rear sprocket, and it can be felt by the rider as a torque on the crankset in the rotational direction opposite to forward pedaling.        Anti-squat refers to the tendency of suspension extension caused by pedaling (i.e., chainstay lengthening) to counteract the suspension compression that would otherwise result from weight transfer associated with the acceleration caused by the same pedaling. Anti-squat is generally given as a percentage, as follows: with 100% anti-squat the extension force caused by chain tension perfectly balances the compression force caused by weight transfer, and the suspension system doesn't extend or compress under pedaling-induced acceleration; and with 0% anti-squat the chain tension does not cause any extension or compression force in the suspension, and the suspension system will compress under acceleration, due to weight transfer alone. A generally accepted approach for quantifying anti-squat is as described in ‘Motorcycle Handling and Chassis Design: the art and science’ (Tony Foale, 2006) and in U.S. Pat. No. 7,128,329. Some anti-squat is generally considered to be desirable. However, too much anti-squat results in resistance to compression of the suspension due to pedal forces when the rear wheel hits an obstacle. Likewise, bump forces are transmitted through the pedals to the rider. In other words, too much chainstay lengthening either reduces the ability of the suspension to react to irregular terrain, or is felt by the rider as movement of the pedals.        Sag refers to the amount of suspension movement under just the static load of the rider. Sag is often used as one parameter when tuning a suspension for a rider. Preload is adjusted until the desired amount of sag is achieved. Nearly all rear suspension systems operate optimally with sag set somewhere between 20-35% of the total suspension travel, depending on the rider's preference and the suspension design. Some sag is considered to be desirable in that it allows the rear wheel to drop into depressions in the terrain, maintaining traction.        Preload refers to the force applied to spring component before external loads, such as rider weight, are applied. The desirable amount of preload is affected by the rider weight and the parameters of the spring components. Preload affects sag; increasing preload reduces sag and decreasing preload increases sag. Thus, adjusting preload affects the ride height of the suspension.        Rebound refers to the rate at which a suspension component returns to its original configuration after absorbing a shock. The term is also generally used to refer to rebound damping, or rebound damping adjustments on shocks, which vary the rebound speed. More rebound damping will cause the shock to return at a slower rate.        Lockout refers to a mechanism to disable a suspension system so as to make it substantially rigid. This may be desirable during climbing or sprinting to prevent the suspension from absorbing power applied by the rider. Some lockout mechanisms also feature a “blow off” system that deactivates the lockout when an appropriate force is applied to help prevent damage to the shock and rider injury under high unexpected loads.        Compression damping refers to a feature that slows the rate of compression in a front fork shock or rear shock. Compression damping is usually accomplished by forcing a hydraulic fluid (such as oil) through a valve when the shock becomes loaded. The amount of damping is determined by the resistance through the valve, a higher amount of damping resulting from greater resistance in the valve. Many shocks have compression damping adjustments which vary the resistance in the valve. Often, lockout is achieved by a compression damping valve that can be adjusted to prevent flow of any hydraulic fluid through the compression damping valve.        Unsprung mass is the mass of the portions of bicycles that are not supported by the suspension systems. For example, in a bicycle with rear and front suspensions, the wheels comprise part of the unsprung mass.        
Input from hard braking often also negatively affected the performance of early full suspension designs. When a rider firmly applied the brakes (which often occurs in terrain situations in which the rear suspension is needed most), some early suspension configurations tended to extend the shock (known as brake jack), causing a stiffening of the suspension, which tends to not allow the suspension to react to bumps very well. Alternatively, some suspension designs exhibit brake squat, where braking forces tend to compress the suspension. This effect, in moderation, can be beneficial to counteract the normal forward weight transfer caused by braking.
As illustrated in the following discussion of some types of prior-art suspensions, rear suspension systems involve complicated interactions of multiple connected components and multiple performance considerations.
One of the simplest and most common bicycle suspension designs is the single-pivot system, in which the rear wheel of the bicycle is attached to the main frame of the bicycle by a single swingarm (often a generally triangular component and often referred to as the rear triangle) pivoting about a pivot located on the main triangle. In simple terms, a lower forward corner of the rear triangle is pivotally attached to the main frame, a lower rearward corner houses the rear wheel axle, and the third upward corner actuates a shock absorber interposed between the third upward corner and the main frame. The pivotal attachment between the rear triangle and the main frame is typically located above the bottom bracket shell. With the single-pivot design, the rear wheel absorbs bumps from irregular terrain by moving in a simple curve (i.e., a circular arc) about the pivot.
With single-pivot suspensions, pedaling forces tend to extend or compress the rear suspension, depending on whether the pivotal attachment between the rear triangle and the main frame is above or below the chain line. Likewise, when a single-pivot suspension system compresses when hitting an obstacle on the trail, or extends when riding over a depression in the ground surface, unwanted forces are exerted on the bicycle riders legs via the pedals. In single-pivot designs in which the pivot is below the chain line, pedaling induced chain tension translates into a force tending to pull the swingarm upwards (i.e., tending to compress the suspension); and by the same token, terrain-induced compression and extension of the suspension tend to dynamically affect chain tension, with compression decreasing chain tension and extension increasing chain tension (i.e., chainstay lengthening), both of which affect the pedaling resistance experienced by the rider. By contrast, in single-pivot designs in which the pivot is above the chain line, pedaling induced chain tension translates into a force tending to pull the swingarm downwards (i.e., tending to extend the suspension); and, again, terrain-induced compression and extension of the suspension tend to dynamically affect chain tension, with compression increasing chain tension and extension decreasing chain tension, both of which again affect the pedaling resistance experienced by the rider.
Generally, the greater the amount of suspension system travel in a single-pivot suspension, the greater these pedaling-induced and terrain-induced effects. Configuring a single-pivot suspension so as to provide a desirable amount of chainstay lengthening (i.e., anti-squat for efficient pedaling), results in too much chainstay lengthening when the suspension system is fully compressed. Lowering the pivot to reduce the total amount of chainstay lengthening when the suspension is fully compressed results in power loss when pedaling, because pedaling forces act to compress the suspension. Shock absorber damping was introduced to reduce the suspension motion induced by pedaling forces. However, shock damping may resist unwanted movement of a suspension, but damping also reduces the ability of the suspension to absorb bumps. Therefore, typically with single-pivot suspension systems, some of the rider's energy is undesirably expended in compressing or extending the suspension, the effectiveness of the suspension is reduced by damping on the shock, and some amount of the rider's energy is dissipated in the shock absorber.
More complicated suspension designs were developed in an attempt to overcome some or all of the single-pivot systems' performance shortcomings. All such suspension systems use a configuration of linkages that is more complicated than a mere single pivot and that generally provide for an axle path of travel during suspension compression and extension that is other than the simple curve about the pivot point achievable typical of single-pivot suspensions.
A popular linkage suspension design is shown in FIG. 3 in U.S. Pat. No. 5,899,480 (commonly referred to as a Horst Link suspension system after the inventor, Horst Leitner). The Horst Link suspension system comprises four pivotally connected linkage members. The first linkage member is the front triangle of the bicycle. The second linkage member is a long swingarm similar to a single pivot's swingarm (i.e., analogous to a chainstay). The third linkage member is analogous to seatstays. The fourth linkage member is located between the third linkage member (i.e., the seatstays) and the first linkage member (i.e., the front triangle). The rear wheel is mounted at the lower end of the seatstays The Horst Link suspension is intended to reduce the amount by which pedaling forces actuate the suspension (and likewise reduce feedback from suspension movement to the pedals) by distributing pedaling forces across both the lower swingarm and the third linkage member.
However, to achieve this goal, the chainstay/swingarm pivot on the front triangle is typically located lower and closer to the bottom bracket, as compared to single-pivot suspension systems. It's in this location to take advantage of the fact that since pedaling forces affect suspension movement less, a low main pivot location does not result in pedal forces compressing the suspension system as much as in single pivot locations. This allows the suspension to move more freely when pedaling forces are applied, and likewise results in less feedback to the rider's legs when the suspension is compressed. These effects are a result of the low main pivot location, which reduces the amount of chainstay lengthening. If a higher main pivot location is employed to achieve an advantageous amount of chainstay lengthening, the path of the rear axle is up and away from the bottom bracket as in single-pivot suspension systems, resulting in too much chainstay lengthening when the suspension is fully compressed.
Because typical Horst Link suspension systems are designed to reduce pedaling effects on the suspension, they generally do not provide the benefit associated with chainstay lengthening (i.e., anti-squat). Since suspension stiffening byway of chainstay lengthening is minimal or non-existent, the acceleration-induced weight shift of the rider toward the rear of the bicycle compresses the rear suspension, resulting in loss of the rider's energy. In practice, many Horst Link suspension designs use a shock absorber with damping to reduce compression of the suspension due to acceleration forces, reducing the ability of the suspension to react to bumps or depressions in the trail.
Dual short-link designs are a popular type of four-bar linkage suspension systems comprising two short links interposed between the bicycle main frame and a rigid rear triangle to which the rear wheel is mounted. A characteristic of dual short-link designs is that they use the relatively high angular velocity of the short links to manipulate the path of the rear axle during suspension compression and extension. However, the relatively high angular velocity of the short links also tends to induce rapid rates of change in the shock leverage ratio. This means that the shock is compressed at varying rates while the rear wheel moves at a constant rate. This discontinuity complicates suspension design; if the rate of change of leverage ratio is too rapid, or the difference between the highest and lowest leverage ratio is large, suspension performance suffers.
To compensate for rapidly changing leverage rates, dual short-link designs are typically configured to provide: a specific amount of sag when the suspension is statically loaded, and a shock absorber finely tuned to match the rear suspension's leverage ratio changes. If sag is not set correctly on dual short-link suspension bikes, the shock absorber is “out of tune” with the leverage ratio applied by the dual short links, and the suspension will not operate optimally. Part of that “tuning” can include not using the first third of suspension travel during the compression stroke, which avoids one part of the rapidly changing leverage rate applied to the shock absorber. By ignoring the first third of the suspension travel, suspension designers can “hide” poorly performing parts of the shock leverage ratio curve, where it is thought by some to have minimal adverse effects on suspension travel.
Dual short-link bikes often do not use sag to full advantage. In practice, many riders cannot or do not set a desirable sag, resulting in sub-optimal performance of the dual short link suspension.
The first widely successful dual short link design is called the Virtual Pivot Point suspension (or VPP), disclosed in U.S. Pat. No. 6,206,397. This suspension system employs two short linkages that move in “opposite directions” so as to manipulate the rear wheel axle path into an S-curve. This design uses chain tension to “hold” the rear axle at its sag point, or the point where the chainstays are at their “shortest length”. Chainstay lengthening occurs both above and below the sag point. As a result, in the VPP design, chain tension impedes suspension extension and thus impedes the rear wheel from dropping into depressions in the terrain, which may adversely affect traction of the rear wheel over irregular terrain.
A dual short link design that employs links pivoting in the same direction is disclosed in U.S. Pat. No. 7,128,329 (Weagle). This design uses anti-squat properties generated by large amounts of chainstay lengthening to counteract the rearward weight shift and resulting compression of the suspension due to acceleration forces. By focusing on anti-squat behavior throughout the suspension travel, this design essentially has an up and rearward axle path through the majority of its travel. This causes chainstay lengthening throughout most of the travel, resulting in chain growth's associated problems, similar to high-pivot single-pivot designs.
Many of the patented dual short link suspension designs featuring two short links rotating in the same direction emulate the function of Weagle's or the VPP designs in various ways, but differ with respect to the placement, length and pivot locations of the two short links. The chainstay lengthening effects are derived from the placement of the links and pivot points. However, by emulating Weagle's or the VPP design, the majority of dual short-link designs have similar performance issues as single pivot designs, relying on a specific amount of suspension sag, not taking full advantage of suspension sag, and highly variable shock leverage ratios.
A suspension design that illustrates the variety of performance considerations that influence innovation in the field of bicycle rear suspensions is the design described in U.S. Pat. No. 7,556,276 (Dunlap), which is directed to performance considerations completely different from those discussed above (e.g., chainstay lengthening, anti-squat etc.). As set out in the Summary of Invention section of the Dunlap patent, the Dunlap design is primarily aimed at lowering the center of gravity of the bicycle and providing a skid plate that helps to actuate the rear wheel.
Approaches to dealing with chainstay lenghthening include the use of an idler wheel. For example, in UK Patent Application GB 2,454,021 (McGrath), the disclosed suspension produces significant undesirable chainstay lengthening, such that an additional component, essentially an idler wheel, is a preferred part of the system. The partial paragraph at the top of page 5 of McGrath reads in part: “Preferably a pivot jockey assembly 41, would be mounted to the main frame to prevent chain deraillement and to minimise the effects of the increasing length of the chain stay 70, (as shown in FIG. 2) when moving through its travel.”